1. Field of the Invention
The present invention relates to ultrafine particles, and to a method and an apparatus for producing the same. As used herein, the term “ultrafine particle” refers to fine particles having an average particle size ranging from 1 nm to 50 nm, that is, so-called “nano size particles” (occasionally referred to as “nano particles”).
2. Description of the Related Art
Conventionally, various methods of producing ultrafine particles have been researched. Since nano particles having an average particle size of below dozens of nano meters cannot be obtained by grinding, build-up methods have been used. Primary examples of build-up methods can be divided into gas phase methods, such as condensation by evaporation and vapor phase reaction, and liquid phase methods, such as precipitation and desolvation.
As an example of a liquid phase method, Japanese Patent Application Laid-Open (JP-A) No. 2000-54012 discloses a method of forming, through reduction, magnetic nano crystals made of metals, intermetallic compounds and alloys. Although it is possible to obtain ultrafine particles having a relatively uniform particle size, the method has limitations in that raw materials must be dissolved as a solution and only a compound having a stoichiometric composition ratio is obtained.
When the gas phase method is used, a thin film can relatively easily be formed. However, various procedures are needed to recover a product in the form of nano particles. For example, JP-A No. 2001-35255 introduces silver or oxygen using in-gas evaporation to obtain nano particles of silver oxide. The Journal of Crystal Growth 45 (1978), pp. 490 to 494, proposes a method of producing ultrafine particles of higher purity by vacuum evaporation on a rheological oil surface (VEROS method). In this method, resistance overheating or an electron beam is used as the evaporation source. However, although these methods are suitable for forming particles made of a single-element of noble metals, it is difficult to produce ultrafine particles made of two or more metal elements. It is also the case that there is no effective way of producing nano particles of alloys having an arbitrary composition.
In the VEROS method described above, resistance overheating, an electron beam or the like is used for vaporization. In this method, when a multi-element base target is used, a problem arises in that, timing of evaporation may shift depending on a difference in a vapor pressure, whereby only giving single-element particles and failing to produce composite particles. In order to solve this problem, it is conceivable to use ion beams as vaporizing means to vaporize a multi-element base target having the form of molecules or an alloy. However, in this case, vaporization efficiency is poor and the apparatus cost is high. Further, these methods require a relatively high vacuum. In order to individually disperse fine particles, it is necessary to either discharge the vaporized molecules from the system before aggregation occurs or protect the surface, and for this purpose, a medium that adheres to the surfaces of particles must be present in the vacuum system. Consequently, there are problems in that the degree of vacuum is decreased and the vaporizing means cannot function in an ordinary manner.
As stated above, it is difficult to obtain nano size particles having a variety of compositions and applicable to various objects by liquid phase methods, and such nano size particles cannot be obtained by gas phase methods such as a CVD method. Because particles aggregate to form a film within one to several seconds even if ordinary sputtering is used, there is currently no specific method capable of producing nano size particles.
Moreover, when arranging nano particles in the form of a film, it is effective to coat a colloidal solution of nano particles. However, it is necessary to select a dispersing medium and a coating equipment suited for particle constituent elements. Although gas phase methods are known in which fine particles are recovered by causing vaporized particles to adhered to a dispersion medium, there is the problem that the particles easily coagulate when their concentration is high. There has also been a demand for a method of readily producing a stable colloidal solution of nano particles.